1. Field of the Invention
The present invention relates to an interference filter for reflecting discrete wavelengths in the vacuum ultraviolet region of the spectrum.
2. The Prior Art
When broad band radiation is incident on a grating monochromator, the output wavelength .lambda..sub.1 (fundamental or first order) is contaminated with integrally related harmonics .lambda..sub.n =.lambda..sub.1 /n which are diffracted in the same direction. Typically, the strongest of these harmonics occurs when n=2 and is called the second order. The presence of these harmonics complicates vacuum ultraviolet (VUV) spectroscopies using synchrotron radiation because the source spectrum extends continuously over many harmonics. The photon energies in the VUV range, 10 to 100 eV (approximately 1200 .ANG. through 120 .ANG.) are difficult to process because most filter materials are too absorbent to be useful, unlike interference filters used in both transmission and reflection modes in the visible portion of the spectrum. Multiple layer mirrors, which also employ interference, are used at larger energies where absorption is sufficiently weak that radiation can penetrate several layers.
Bandpass reflectors in the long wavelength region of the VUV (roughly 1000-2000 .ANG.) are disclosed in U.S. Pat. No. 4,408,825 to Stelmack and U.S. Pat. No. 4,714,308 to Sawamura et al. Both of these reflectors employ multiple layers and are concerned with filtering out an entire portion of the VUV region by sharply attenuating radiation having wavelengths longer than VUV.
One approach to decontaminating radiation at higher energies, i.e., greater than 40 eV (less than 310 .ANG.), consists of using two or three critical angle mirrors positioned in a predetermined relationship. These critical angle mirrors exploit the wavelength dependence of the critical angle to absorb harmonics while reflecting the fundamental wavelength. At lower energies (higher wavelengths), the relative amount of fundamental radiation reflected compared to the amount of harmonics absorbed is too low for many applications. Further, since at least two or three reflections are required, losses are greater than those for a scheme requiring only a single interaction.
Presently, the only means of reducing unwanted radiation in the 10 to 100 eV range is by using transmission filters constructed of thin, free standing metallic films. These transmission filters rely on a limited number of absorption features associated with the electronic states of their constituent atoms in order to attenuate undesired photon energies. The fixed energies of these absorption features means that these features are only useful for certain first order-second order combinations of photon energy and do not suppress higher orders. Transmission filters are also extremely fragile, can be destroyed during pump down or venting of the vacuum system in which they are used, and are not tunable since they only work for specific first order-second order combinations. Further, when used in conjunction with a monochromator, these filters represent an additional source of attenuation of photons having the desired energy.